1. Field of the Invention
The present invention relates to a sheet folding device that performs a sheet folding process on a sheet-like member (hereinafter, simply referred to as a “sheet”) such as a sheet of paper, transfer paper, or an overhead projector (OHP) sheet; an image forming system that includes the sheet folding device and an image forming apparatus such as a copying machine, a printer, a facsimile, or a digital multi-function peripheral; and a sheet folding method that is performed in the sheet folding device.
2. Description of the Related Art
With regard to a so-called post-processing peripheral that performs a predetermined process on a sheet discharged from an image forming apparatus such as a copying machine, Japanese Patent Application Laid-open No. 2004-210436 or Japanese Patent Application Laid-open No. 2000-143081 discloses the technology of performing center-binding on a plurality of sheets with a staple and folding the sheets in half (hereinafter, referred to as “center-binding fold”) by pressing the sheets using pressing rollers or pressing plates.
In particular, Japanese Patent Application Laid-open No. 2004-210436 discloses an invention of the booklet forming device that performs a half-fold pressing process on a set of sheets. The booklet forming device includes: a push blade that pushes a folding-line position of paper placed on a sheet table in the direction perpendicular to a surface of the paper; a stopper plate that receives the paper pushed by the push blade; and a pair of press blades that are disposed at both sides of a movement path of the push blade and are driven obliquely from a standby position toward a push blade landing position on the stopper plate. Then, the paper is made to be abutted on the stopper plate at the folding-line position by the push blade while the paper is kept to be folded in half, and both sides of the folded portion of the paper are pressed by the press blades so as to form the spine of a booklet.
Furthermore, Japanese Patent Application Laid-open No. 2000-143081 discloses an invention of the sheet post-processing device that binds a sheet bundle at the center position in the conveying direction, operates a folding blade at nearly the same position as the folding position of the bound sheet bundle so as to fold the bound sheet bundle and makes a bound booklet by enhancing the folded state of the sheet bundle by being nipped by a pair or a plurality of pairs of press rollers while the bound sheet bundle is being conveyed. The sheet post-processing device includes a cutting unit that evenly cuts the edge portion of the center-bound booklet on the opposite side of the bound side and that is provided at the upstream position with respect to a roller serving as the final press roller among the press rollers in the conveying direction of the center-bound booklet, and sets a position of the center-bound booklet to be cut to the cutting position of the center-bound booklet while at least part of the press rollers nip the bound sheet bundle, thereby to perform a cutting operation.
Furthermore, Japanese Patent Application Laid-open No. 2010-6602 discloses, with the aim of providing a center-binding folding device capable of forming a center-bound and -folded booklet having a sharp crease quickly, a center-binding-folding mechanism that includes: a folding unit that folds a sheet bundle in half; a pressing unit that stops the half-folded sheet bundle at a predetermined position and applies pressure to the crease of the stopped sheet bundle from the front and rear faces of the sheet bundle interposed between the pressing surfaces facing each other; and a pressing number control unit that determines the number of times the pressing unit is to perform a pressing operation for applying pressure to the crease of the sheet bundle and that controls the pressing unit so that the pressing unit performs the pressing operation for the determined number of times.
However, when the press-folding operation is performed by pressing and stroking the paper (sheet) bundle with the leading end of the press blade as in the invention disclosed in Japanese Patent Application Laid-open No. 2004-210436, there has been a problem in that damage such as a scratch or wrinkle may occur on the surface of the center-bound paper (sheet) bundle due to the pressing or stroking.
Furthermore, when the folding operation is performed by the pair of folding rollers as in the invention disclosed in Japanese Patent Application Laid-open No. 2000-143081, in the sheet folding mechanism that inserts the sheet or the sheet bundle from the creasing position into the nip between the pair of folding rollers using the folding blade to fold and align the sheet bundle, a wrinkle folding phenomenon occurs at the trailing edge side of the sheet when the sheet or the sheet bundle is folded and aligned at the creasing position by the folding roller. At this time, because the folding roller has a round shape, it is difficult to keep the roller pressure across the folding roller uniform while the sheet bundle is conveyed by the pressure applied by the folding roller through the way from the folding operation to the completion of the conveying operation. Therefore, the deformation or the displacement has been accumulated in the sheet bundle due to the non-uniform roller pressure, thereby to cause a wrinkle on the sheet bundle.
Because uniformity and balancing are required for the pressure of the folding roller, as described above, there has been a problem in that the precise configuration of the folding roller in the parallelism between the circumference and the axis of rotation of the folding roller makes the production of the folding roller difficult and costly, the weight thereof is heavy due to the increased rigidity, and a high level of skill is required for the adjustment thereof.
Furthermore, the invention disclosed in Japanese Patent Application Laid-open No. 2010-6602 is configured such that, after the folding operation is performed by the folding roller, the fold increasing operation is performed with a pressing surface at the pressing unit provided in the downstream of the folding roller in the conveying direction. However, the paper is folded with the folding roller; the folding roller is very expensive, the weight is heavy due to the increased rigidity, and a high level of skill is required for the adjustment as in the invention disclosed in Japanese Patent Application Laid-open No. 2000-143081. In addition, the invention relates to a mechanism that performs a process of further applying pressure to the folded bundle with the pressing surface after the folding process, and the requirement of performing an additional process for applying the pressure to the folded bundle after the folding process inevitably reduces the productivity. The reduction in the productivity becomes more apparent when the pressing operation is performed a plurality of times in accordance with the number of sheets.
Therefore, there is a need to form a crease with high quality using a low-cost mechanism without causing damage such as a wrinkle or a scratch on the sheet or a reduction in the productivity.